Automotive radar solutions for advanced driver assistance systems (ADAS) are currently being deployed on a large scale. These solutions can typically be grouped into long range radar (LRR) applications and short range radar (SRR) applications. Both of these applications generally use frequency modulated continuous wave (FMCW) modulation techniques in order to be able to identify a radar target, such as a car or a pedestrian. In such radar systems, multiple receivers and receiver channels (e.g. 2-16) are connected to an antenna array or antenna patch with separate antenna elements so that any phase difference between signals on the receiver channels provides an indication of angle information of the radar target.
These radar systems typically use millimeter wave (MMW) frequencies for transmission and reception. The frequency synthesisers, comprising voltage controlled oscillators (VCOs) that are responsible for the generation of the millimeter wave frequencies, are important to the operation of the radar systems. Generally, voltage controlled oscillators operating at millimeter wave frequencies need to present a low phase noise, whilst providing a wide tuning range in order to cover the required modulation band (e.g. 1 GHz for LRR and 4 GHz for SRR).
Direct digital synthesis (DDS) is a very useful technique that has gained favour in the generation of radio frequency signals for use in a variety of applications from radio receivers to signals generators and particularly in the automotive radar field. DDS generates the waveform directly using digital techniques, rather than the traditional way adopted by indirect synthesizers that use a phase locked loop as the basis of their operation. The use of DDS has become more widespread in recent years with the advances being made in integrated circuit technology, which allows much faster speeds to be handled that, in turn, enable higher frequency DDS chips to be made. DDS is often used in conjunction with indirect or phase locked loop (PLL) synthesizer loops. By combining both technologies it is possible to take advantage of the best aspects of each.
Some DDS operate by storing the points of a waveform in digital format, and then recalling them to generate the waveform. Other DDS generate a sine wave without storing the points. The rate at which the synthesizer completes one waveform then governs the frequency, for example advancing the waveform can be viewed as a phase signal progresses around a circle. The synthesizer operates by storing various points in the waveform in digital form and then recalling them to generate the waveform.
Referring to FIG. 1, U.S. Pat. No. 6,569,607 describes a known radar system 100 whereby a built-in system test circuit 150 is included to calibrate a receiver to determine a phase difference between the transmit signals 140, 142. The built-in system test circuit 150 includes a digital (10) up-mixer 152 with externally-generated analog intermediate frequency (IF) signals 102, 104 routed to the digital (10) up-mixer 152. Furthermore, built-in system test circuit 150 includes a 77 GHz local oscillator input signal 120 that is amplified by amplifier(s) 127 and input to the IQ up-mixer 152. The up-converted high frequency signals output from the IQ up-mixer 152, which are in effect multiplied representations of the externally-generated analog intermediate frequency (IF) signals 102, 104 with the 77 GHz local oscillator input signal 120, are coupled into the respective transmitter paths via radio frequency couplers 110, 112. In this manner, the externally generated analog intermediate frequency (IF) signals 102, 104, multiplied by the 77 GHz local oscillator signal 120 form test-transmitter signals that aim to represent transmitter signals being received at receiver input ports 106, 108. The test-transmitter signals are then routed through respective baluns 114, 116 and input to respective digital (10) down-mixers 126, 128, where the test-transmitter signals are multiplied with a 77 GHz local oscillator signal 120 that is amplified by amplifier(s) 122, 124 respectively. The respective low frequency (e.g. IF) outputs from the respective digital (10) down-mixers 126, 128 are input to low frequency processing circuits 130, 132 that process the signals to determine the phase difference between the transmit paths 140, 142, and calibrate each receiver path accordingly.
As is apparent, the technique described in U.S. Pat. No. 6,569,607 requires additional radio frequency (RF) circuitry, as well as notably provided from an external test generation source that impacts on die area and power consumption. Furthermore, U.S. Pat. No. 6,569,607 requires more complexity and provides less flexibility, in particular due to a need for externally-generated digital (10) IF signals 102, 104 to be used and input to the built-in system test circuit 150.
In the radar field, in order to be ISO26262 compliant, there has been a recent requirement to detect errors and problems, such as a transmitter phase imbalance, on-chip.